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Shape Fabrics and Superimposed Simple Shear Strain in a Precambrian Shear Belt, W Greenland

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Shape Fabrics and Superimposed Simple Shear Strain in a Precambrian Shear Belt, W Greenland J. geol. Soc. London, Vol. 136, 1979, pp. 471-488, 12 figs. Printed in Northern Ireland. Shape fabrics and superimposed simple shear strain in a Precambrian shear belt, W Greenland John Grocott SUMMARY: Shapefabric type in thenorthern part of the IkertBq shearbelt and the adjacentarea has been recordedboth qualitatively andquantitatively, and its variationis described. Shape fabric variation is shown to be closely related to structural changes, and both are reconciled with a deformation model involving superimposed strain. In theshear belt itselffinite strainshape fabrics are shown tobe aconsequence of superimposition of 2 differently orientated simple shear strains. The earlier simple shear strain initiated the shearbelt and was characterized by horizontal ENE-WSW movements in avertical shear plane. The second deformation was produced by overthrusting in a SSE direction. In effect the XZ plane of the later deformation was superimposed on YZ sections through shape fabrics produced by transcurrent movements. The overthrusting deformation was superimposed on earlier structures with gradually increasing intensity, allowing the resultant deformation path to be recorded in detail. A steep strain gradient marks the northern boundaryof the shear belt, butlow deformation of shear belt age canstill be recognized further N. Shape fabric variation N of this limit of intense strain is described.Structural and shape fabric variationin this area is aconsequence of superimposition.. of low magnitudes- of overthrusting simple shearstrain on pre-shearbelt structures and fabrics. The IkertBq shear belt is a major zone of intense Nag. 2 respectively, separated by a swarm of basic ductilestrain in the Precambrianbasement gneiss dykes, called Kangfimiut dykes. Inthe W, strong complex of western Greenland. Compelling evidence Nag. 1 deformation is recognized across much of the accumulated during earlier work in the shear belt has shear belt, but not in an area of low strain centred on led to the adoption of a simple shear model as a basis Kingaq, orat the extreme northern margin of the for its structural interpretation (Escher & Watterson shear belt. Nag. 2 strain is absent immediately S of the 1974; Escher et al. 1975). Despite a weight of sup- Kingaq augen in the Itivdleq district, but increases porting evidence, shape fabrics in the shear belt are gradually northwards culminating in a 7 km wide zone generally not of the plane strain type demanded by of intense deformation along the northern boundary this model. This paper describes shape fabric variation (Fig. 1). Apart from the boundary of the shear belt SE in theshear belt, together with the accompanying of SandreStr~mfjord, where coincident facies and structural changes. An important aim is to describe deformation boundaries mark the southern limit of a how shape fabrics and structures evolve in large scale zone of Nag. 2 strain, the distributionand relative shear belts as a result of more than one deformation. intensity of the component deformations in the rest of Trending ENE-WSW, the IkertBq shear belt has an the shear belt are unknown. exposed length of 150 km to the inland ice, and is of Ramsay & Graham (1970) showed that fabric ele- variable width (Fig. 1). The dominant lithology is ments formed in simple shear zones reflect displace- granodioritichornblende and biotite-amphibolite ments across such zones. Accordingly, where facies gneiss. The shear belt is flanked Nand S by Kangfimiut dykes are notdeformed, vertical ENE- gneisses which are dominantly granodioritic, with min- WSW, or E-W planar elements of shape fabrics (folia- eral assemblages of the hornblendegranulite facies. tion) coupled with sub-horizontallinear elements These rocks are largely unaffected by deformation (stretching directions) show that movements were dex- responsible for forming the shear belt. Whilst at the tral and sub-horizontal within an approximately E-W southern boundary of the shear belt, facies and defor- vertical shear plane(Bak et al. 1975). Where the mation boundaries coincide, at the coastal part of the dykes are intensely deformed, Nag. 2 strain is charac- northern boundary the steep strain gradient is 5 km N terized by overthrustingmovements within a shear of the facies boundary (Fig. 1).In this 5 km wide zone plane dipping 20"-50"NNW (Escher et al. 1975; the gneisses were reworked in granulite facies, whilst Grocott 1977). elsewhere in theshear beltthey were retrogressed As will be demonstrated, many conclusions are built during reworking (Grocott, in press). on the belief that in rocksnot containing large The IkertBq shear belt occurs within the Nagssugto- amounts of mica, shape fabrics can be used to indicate qidian of western Greenland. The 2 principal phases the orientation and magnitude of the finite strain, and of deformation recognized in it are called Nag. 1 and that they also reflect the ellipsoid type (oblate, prolate 0016-7649/79/0700-0471$02.00 @ TheGeologicalSociety Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/471/4885865/gsjgs.136.4.0471.pdf by guest on 28 September 2021 472 J. Grocott l . ... ' S. .. , 0: - .*'9 i. .e .. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/471/4885865/gsjgs.136.4.0471.pdf by guest on 28 September 2021 Shapefabrics, Precambrianshear belt, W Greenland 473 or whatever) describing the finite strain (Flinn 1965; Shapefabrics in the gneisses are of planestrain Ramsay & Graham 1970). type, belonging to the LS class of the LS fabric system It is useful to distinguish 3 types of macroscopic (Flinn 1965). They give the orientation of the Nag. 1 fabric. Shape fabrics usually consist of quasi-elliptical strain ellipsoid in this area (Fig. 3). aggregates of many mineral grains. Quartz is an excep- In central Sagdlerssuaq both dykes and gneisses are tion at high metamorphic grade, and consists of len- folded. The folds are open with 'S' asymmetry and soid grains made up of relatively few individual grains. verge to the S. They are believed to be Nag. 2 in age. Such differences in texture reflect different deforma- Axes plunge gently W, parallel to theearlier stretching tion mechanisms (White1976). Nevertheless, in the fabric in steeply dipping or vertical axial planes gneisses which arethe subject of this paper,shape (Fig. 24. Nag. 1 shape fabrics arereorientated in fabrics formed by quartz do not differ in orientation these folds, but arenot modified from LS type. In from those formed by other minerals in any one rock. profile section they exhibit a shallowly dipping short Mineral fabrics, unlike shape fabrics, are defined by limb, and a steep long limb with bandingparallel the crystallographic orientation of individual mineral shape fabrics (Fig. 4). This style is important when grains such as biotite and opaques (Watterson 1968). effects of superimposed strain are considered. Where the last deformation is strong, shape and min- eral fabrics are parallel. In contrast,mineral fabrics Area 2 andshape fabrics are not parallel where the last This area includes the 7 km wide zone of intense deformation is weak, showing that mineral fabrics do Nag. 2 deformation at the northern boundary of the not reflect the finite strain. A thud fabric often de- shear belt (Fig. 1). From the southern coast of Sarfan- veloped in gneisses is simply a lithological variation on guaqland strain increases northwards into this zone. a scale greater than that induced by the formation of The most strongly deformed rocks occur on NW Sar- shape fabrics. This fabric is called banding. fanguaqland and then strain begins to decrease north- Throughout this paperprolate shape fabrics are wards. This decrease becomes marked at a deforma- termed L tectonites, fabrics in the constrictional field tion boundary which can be mapped on Umanarssugs- L > S tectonites,plane strain fabrics LS tectonites, suaq andsouthern Manitsorssuaq. Thisboundary fabrics in the flattening field S>L tectonites,and marks the northern limit of high Nag. 2 strain (Fig. l), oblate fabrics S tectonites(after Flinn 1965). The and further N shape fabrics become variably orien- parameter k (Flinn 1965) forthese fabrics takes values tated. It is taken as the northern boundary of area 2 of m, m> k > 1, 1, 1> k >O and 0 respectively. and of the shear belt. Where Nag. 2 deformation is The work reported here relates to the coastal part of most intense the ,foliation strikes 065" and dips 55" the shear belt N of Kingaq, and distinguishes 3 struc- NNW with adown-dip stretching direction. These tural areas. From S to N, area 1 consists of western fabricspermit the orientation of the Nag. 2 shear and centralSagdlerssuaq, area 2 of western Sarfanguaq- plane to be estimated (Fig. 3). The effect of superim- land,southern Umanarssugssuaq and part of south- position of different amounts of Nag. 2 strain on ear- ern Manitsorssuaq, and area 3 of western Manitsors- lier structures and fabrics is described in detail in suaq andnorthern Umanarssugssuaq (Fig. 1). Only sections 2 and 3. areas 2 and 3 are discussed in detail. Area3 Area1 This is N of the steep gradient in Nagssugtoqidian On western Sagdlerssuaq, vertical foliation in strain which crosses Umanarssugssuaq andsouthern strongly deformed gneisses trends 075" associated with Manitsorssuaq and which is taken as thenorthern a shallow westerly-plunging stretchingdirection boundary of the shear belt. Low or moderate Nag. 2 (Figs. 2a,b). The foliation is axial planar to isoclinal strain can nevertheless be recognized at most pre-dyke folds of the banding which plunge parallel to localities. Measurement of shape fabrics across Manit- the stretchinglineation (Fig. 2c). Kanghmiut dykes sorssuaq shows that Nag. 2 strain gradually increases show persistent slight discordances to banding, southwardsas this deformationboundary is ap- trending 5-10' more north-easterly. Many dykes re- proached. This increase progressively modifies earlier tain primary intrusive structures including discordant shape fabrics and structures which are shown to be apopheses, and some have primary igneous textures.
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